OLED based displays have gained significant interest recently for many display applications because of their faster response times, larger viewing angles, higher contrast, lighter weight, lower power, and amenability to flexible substrates, as compared to liquid crystal displays (LCDs).
The simplest way of addressing an OLED display is to use a passive matrix format. Although passive matrix addressed OLED displays are already in the marketplace, they do not support the resolution needed for next generation displays, which use high information content (HIC) formats. HIC formats are only possible with an active matrix addressing scheme.
Active matrix addressing involves a layer of backplane electronics, based on thin-film transistors (TFTs). These thin film transistors provide the bias voltage and drive current needed in each OLED pixel and may be fabricated using amorphous silicon (a-Si:H), polycrystalline silicon (poly-Si), organic, polymer, or other transistor technologies. When compared to passive matrix addressing, active matrix addressing uses a lower voltage on each pixel and the current throughout the entire frame period is a low constant value. Thus, active matrix addressing avoids the excessive peak driving and leakage currents associated with passive matrix addressing. This increases the lifetime of the OLED.
LCDs are electric field driven devices. OLEDs, on the other hand, are current driven devices. Thus, the brightness and stability of the light emitted by a given OLED used in a display is dependent on the operation of the TFTs in the current drive circuit. Thus AMOLED displays are far more sensitive to TFT instabilities including, spatial and temporal variations in transistor threshold voltage, mobility instability, and mismatch issues. These instabilities need to be addressed for widespread use of OLED based displays.
FIG. 1 presents a graph of threshold voltage 'shift vs. stress voltage for various times for amorphous silicon based TFTs. It is readily apparent from FIG. 1 that the threshold voltage of the transistors varies over time. If these transistors were used in a display, the variation in threshold voltage would likely result in variation in the brightness of the OLED across the array and/or a decrease in brightness over time, both of which are unacceptable.
A simple pixel driver circuit is shown in FIG. 2. This “2T” circuit is a voltage programmed circuit. Such a circuit is not practical for OLED displays as such a circuit can not compensate for variations in transistor threshold voltage. One solution to this variation in threshold voltage is to use a current programmed circuit to drive the OLED of the pixels. Current programming is a good method for driving AMOLED displays since the OLED is a current driven device, and its brightness is approximately linearly dependent upon the current flowing through it.
One such current programmed circuit is presented in FIG. 3. This circuit incorporates a current-mirror which compensates for any shift or mismatch in the threshold voltage of the drive transistor T4 which ensures that the brightness of the OLED does not decrease over time. This feature of the circuit allows its drive characteristics to be much improved as compared to the 2T circuit of FIG. 2.
When programming the circuit of FIG. 3, VADDRESS is high and a current IDATA is applied. This current initially flows through transistor T1 and charges capacitor CS. As the capacitor voltage rises, T3 begins to turn on and IDATA starts to flow through T2 and T3 to ground. The capacitor voltage stabilizes at the point when all of IDATA flows through T2 and T3, and none through T1. This process is independent of the threshold voltage VT of transistors T3 and T4.
The gates of T3 and T4 are connected, so the current flowing through T3 is mirrored in T4. This topology allows us to have on-pixel current gain or attenuation depending on the sizing of T3 and T4, so that the respective data current can be proportionately smaller or larger than the OLED current. In an active matrix array, pixels are scanned and programmed in a row-by-row fashion. The time taken to scan all rows (one frame) is called the frame time. During array operation, the switching TFTs (T1 and T2) are ON only once in the frame time.
However, existing current programmed circuits do not adequately address long-term stability in the OLED drive current due to differential Vt-shift and other bias, temperature, or mechanical stress related degradations and mismatches in the current mirror.